In optical disk technologies, data can be read out from a rotating optical disk by irradiating the disk with a relatively weak light beam with a constant intensity, and detecting the light that has been modulated by, and reflected from, the optical disk.
On a read-only optical disk, information is already stored as pits that are arranged spirally during the manufacturing process of the optical disk. On the other hand, on a rewritable optical disk, a recording material film, from/on which data can be read and written optically, is deposited by an evaporation process, for example, on the surface of a base material on which tracks with spiral lands or grooves are arranged. In writing data on such a rewritable optical disk, data is written there by irradiating the optical disk with a light beam, of which the optical power has been changed according to the data to be written, and locally changing the property of the recording material film.
It should be noted that the depth of the pits, the depth of the tracks, and the thickness of the recording material film are all smaller than the thickness of the optical disk base material. For that reason, those portions of the optical disk, where data is stored, define a two-dimensional plane, which is sometimes called an “information storage plane”. However, considering that such an “information storage plane” actually has a physical dimension in the depth direction, too, the term “information storage plane” will be replaced herein by another term “information storage layer”. Every optical disk has at least one such information storage layer. Optionally, a single information storage layer may actually include a plurality of layers such as a phase-change material layer and a reflective layer.
To read data that is stored on a recordable optical disk or to write data on such an optical disk, the light beam always needs to maintain a predetermined converging state on a target track on an information storage layer. For that purpose, a “focus control” and a “tracking control” are required. The “focus control” means controlling the position of an objective lens perpendicularly to the information storage layer (which direction will be referred to herein as a “substrate depth direction”) such that the focus position of the light beam is always located on the information storage layer. On the other hand, the “tracking control” means controlling the position of the objective lens along the radius of a given optical disk (which direction will be referred to herein as a “disk radial direction”) such that the light beam spot is always located right on a target track.
Various types of optical disks such as DVD (digital versatile disc)-ROM, DVD-RAM, DVD-RW, DVD-R, DVD+RW and DVD+R have become more and more popular these days as storage media on which a huge amount of information can be stored at a high density. Meanwhile, CDs (compact discs) are still popular now. Currently, next-generation optical disks, including Blu-ray disc (BD), which can store an even greater amount of information at a much higher density, are under development, and some of them have already been put on the market.
The cross-sectional structures of these optical disks change from one type to another. For example, these optical disks are different in physical track structure, track pitch, and depth of the information storage layer (i.e., the distance from the surface of the optical disk, through which the incoming light enters the disk, to the information storage layer). To read or write data properly from/on these multiple types of optical disks with those various physical structures, the information storage layer of each of these optical disks needs to be irradiated with a light beam with an appropriate wavelength by using an optical system that has a numerical aperture (NA) associated with the specific type of the disk.
FIG. 1 is a perspective view schematically illustrating an optical disk 200. Just for reference, an objective lens (converging lens) 20 and a light beam 22 that has been converged by this objective lens 20 are shown in FIG. 1. The light beam 22 passes through the light-incoming side of the optical disk 200 and is converged onto the information storage layer, thereby forming a light beam spot on the information storage layer.
FIGS. 2(a), 2(b) and 2(c) schematically illustrate cross sections of a CD, a DVD and a BD, respectively. Each of these optical disks shown in FIG. 2 has a principal surface (i.e., light-incoming side) 200a and a back surface (i.e., a label side) 200b and includes at least one information storage layer 14 between these surfaces. On the back surface 200b of the optical disk, arranged is a label layer 18 on which the title, graphics, and so on have been printed. Any of these optical disks has an overall thickness of 1.2 mm and a diameter of 12 cm. For the sake of simplicity, pits, grooves and other unevenness are not shown in FIG. 2 and the reflective layer is not shown there, either.
As shown in FIG. 2(a), the CD's information storage layer 14 is located at a depth of about 1.1 mm as measured from the principal surface 200a. To read data from the CD's information storage layer 14, a near-infrared laser beam (with a wavelength of 785 nm) needs to be converged such that its focal point is located right on the information storage layer 14 by focus control. The objective lens for use to converge the light beam needs to have a numerical aperture (NA) of approximately 0.5.
As shown in FIG. 2(b), the DVD's information storage layer 14 is located at a depth of approximately 0.6 mm as measured from the principal surface 200a. In an actual DVD, two substrates, each having a thickness of approximately 0.6 mm, are boned together with an adhesive layer. In an optical disk with two information storage layers 14, the respective distances from the principal surface 200a to the information storage layers 14 are approximately 0.57 mm and approximately 0.63 mm, respectively. That is to say, those two information storage layers are located very close to each other. That is why only one information storage layer 14 is shown in FIG. 2(b), no matter how many information storage layers 14 are actually included. To read and write data from/on the DVD's information storage layer 14, a red laser beam (with a wavelength of 660 nm) needs to be converged such that its focal point is located right on the information storage layer 14 by focus control. The objective lens for use to converge the light beam needs to have a numerical aperture (NA) of approximately 0.6.
As shown in FIG. 2(c), the BD includes a thin coating layer (light transmitting layer) with a thickness of approximately 100 μm on the principal surface 200a and its information storage layer 14 is located at a depth of about 0.1 mm as measured from the principal surface 200a. To read data from the BD's information storage layer 14, a blue violet laser beam (with a wavelength of 405 nm) needs to be converged such that its focal point is located right on the information storage layer 14 by focus control. The objective lens for use to converge the light beam needs to have a numerical aperture (NA) of approximately 0.85.
Currently, these various types of optical disks are on the market and used extensively. Under the circumstances like these, a single optical disk drive should read from, and write to, as many types of optical disks as possible. For that purpose, the optical disk drive should include a light source and an optical system, both of which can deal with multiple types of optical disks, and should appropriately recognize the type of the optical disk that has been loaded into the optical disk drive.
The optical disk drive disclosed in Patent Document No. 1 recognizes the type of the given optical disk by optically detecting the depth of the information storage layer of that optical disk. Portion (a) of FIG. 3 schematically illustrates how the gap between the principal surface 200a of the optical disk 200 and the objective lens 20 decreases gradually. This optical disk 200 includes a substrate 12, which is transparent to a light beam, an information storage layer 14 that has been formed on the substrate 12, and a protective layer (coating layer) 16 that covers the information storage layer 14. The optical disk 200 illustrated in portion (a) of FIG. 3 corresponds to a BD and the coating layer 16 has a thickness of about 0.1 mm. There is a label layer 18 on which an image, characters and so on are printed, on the back surface 200b of the optical disk. It should be noted that the thickness of the label layer 18 is not to scale.
Portion (a) of FIG. 3 illustrates a situation where the focus position of the light beam 22 is located on the surface 200a of the optical disk, a situation where the focal point of the light beam 22 is located on the information storage layer 14, and a situation where the focal point of the light beam 22 is located inside the substrate 12. Portion (b) of FIG. 3 schematically shows a focus error (FE) signal to be generated when the focus position of the light beam 22 varies with time. The FE signal changes so as to draw a small S-curve when the focal point of the light beam 22 passes the surface 200a of the optical disk 200. On the other hand, when the focal point of the light beam 22 passes the information storage layer 14 of the optical disk 200, the FE signal changes so as to draw large S-curves. Portion (c) of FIG. 3 schematically shows the amplitude of a radio frequency (RF) read signal to be generated when the focus position of the light beam 22 varies with time. It can be determined that the focal point of the light beam 22 is located on the information storage layer 14 when the amplitude of the RF signal shows a non-zero significant value and when the FE signal goes zero. If the focus servo is turned ON in such a situation, the position of the objective lens is controlled such that the FE signal is always equal to zero. Such an operation of turning the focus servo ON around the center of the S-curve of the FE signal (i.e., near the zero-cross point of the FE signal) when the S-curve is detected while a focus search is being carried out in search of the information storage layer 14 will be referred to herein as a “focus finding operation”.
The position of the objective lens when the S-curve of the FE signal is detected can be determined by reference to the value or magnitude of the electrical signal being supplied to the actuator that is controlling the position of the objective lens. As a result, the depth of the information storage layer 14 can be detected, and eventually, the type of the given optical disk can be recognized by the depth of the information storage layer 14.    Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2004-111028